Keep existing-call pipeline registration open during admission drain

When a shared worker supervisor drains, close admission for new root sessions while allowing already-admitted roots to register later media or transfer pipelines. Preserving running futures alone does not preserve a call: a global non-ready registration guard can reject its next phase and clean up the new pipeline. Test draining between the primary phase and a later phase on the same root, asserting that new roots are rejected while the existing root can continue. This condition depends on supervisor ownership; isolated child supervisors may not share the parent’s drain state.

Applies when: calls or sessions create additional pipelines after initial admission. Keywords: drain admission shared supervisor existing session transfer pipeline registration